Preparation of soil extracts for available nutrients

Plants draw their nutrients from air, water and soil. The bulk of mineral nutrients come from soil. Soil available form of nutrient is that fraction which is distributed in different discrete chemical forms, which often exist in a state of dynamic equilibrium and constitute the pool from which plants draw it. Soil available form of a nutrient is also that fraction whose variation in-amount is responsible for significant changes in yield and responses. The nutrient available to biological organisms is termed as bio available nutrient. Available nutrients in the soil are water soluble and exchangeable. The reagent used for the extraction of this pool simulates the plant roots and extracts the same amount of available nutrient as the plant is able to take in. The following are various extractants for the available fraction of nutrients.

Preparation of soil water extract for water soluble nutrients.

The water soluble nutrients, which consist of cations and anions in different proportion are Ca², Mg^2-, K⁺, Na⁺, NH₄^–, Fe²⁺, Mn²⁺, Cu²⁺, Zn⁺², SO₅^-2, NO₃^–, Cl^–, MoO₄^-2, and BO₃^3-. These are nutrients that can be extracted by using distilled water with soil water ratio of 1:5.

Procedure

Weigh 100 gm of soil passed through 2 mm sieve into one litre reagent bottle with a cork.
Add 500 ml of distilled water and small quantity of charcoal (decolourising agent) and shake the contents by end over end for 30 minutes to get a clear extract.
Filter the contents through Whatman No. 1 filter paper.
Store the extract in a clean bottle for the estimation of water-soluble nutrients in the soil.
Add 2 to 3 drops of TEA (Tri Ethanol Amine) to this extract to prevent algal growth and place in a refrigerator.

Preparation of chemical extracts for water soluble + exchangeable nutrient

There are a variety of extractants used for extracting available nutrients from soils.

Available P: Available P content in the soil is extracted by using two extractants depending on soil pH.
Olsen’s reagent: 0.5 M NaHCO₃ solution of pH 8.5. This reagent is used for extraction of P from slightly acidic, neutral, alkaline and calcareous soils.
Bray’s reagent: 0.03 M NH₄F + 0.025 N HCI solution. This is used as an extractant for soils with acidic pH.
Available K: Available K is extracted by using neutral normal ammonium acetate (CH₃COONH₄) solution.
Available S: It is extracted by using CaCl₂ (0.15 %) solution,
Available micronutrients: The available micronutrient (cationic or anionic).

a) For extracting cationic micronutrients (Zn, Cu, Fe and Mn)

DTPA (Dicthylene Triamine Penta Acetic acid) reagent is used. DTPA extractant contains 0.005 M DTPA + 0.01 M CaCl_{2 .}2H₂O + 0.1 M TEA. Weigh 1.967g DTPA salt and 1.47g CaCl₂. 2H₂O salt in a beaker. To this add 20-25 mL of glass distilled water and 13.3 mL of triehanolamine followed by 100 mL of glass distilled water. Transfer to 1 L volumetric flask, adjust the pH to 7.3 with dilute HCI (1:5) and make the volume to one liter with glass distilled water.